<?php
$str = '我是一只 jlasjdflajsdlfjalsdf<more> sdfjsaldkjflsajdflajsdlfj';
$more = '<more>';
$from = 0;
$toMore = strpos($str, $more);
echo substr($str, 0, $toMore)."<br/>";
echo strstr($str, $more);
echo $toMore;
echo preg_replace('#^(?:[\x00-\x7F]|[\xC0-\xFF][\x80-\xBF]+){0,'.$from.'}'.
                       '((?:[\x00-\x7F]|[\xC0-\xFF][\x80-\xBF]+){0,'.$toMore.'}).*#s',
                       '$1',$str);
?>